The remote measurement of atmospheric wind profiles to provide instantaneous and trend data has become a requirement of paramount importance at the major airports of the world. This need for wind profile data has been tragically demonstrated by numerous aviation disasters caused by wind anomalies such as wind shear resulting from weather related phenomena or vortices created by heavy aircraft.
Attempts have been made to provide instantaneous wind profile data but they require systems which are complex, costly and inaccurate or impractical. Doppler radar is one such system; another solution lies in the application of electro-acoustic techniques which involve transmitting an acoustic signal and detecting scatter echoes resulting from wind movements in a narrow zone. Such a system, using sound as the energy probe, has been termed "sodar". Since the propagation speed of radar waves is approximately 882,000 times that of sound waves, using sodar, it is considerably less difficult to achieve the required high resolution than using radar. Consequently, a high resolution Doppler radar for local wind shear monitoring would tend to be significantly more expensive than a sodar system for the same purpose.
C. Pear, Jr., U.S. Pat. No. 3,379,060 for "Wind Meter" issued Apr. 23, 1968 is an early example of the use of sonic transmission to determine wind velocity. In systems such as this, a single transducer or loudspeaker radiates sonic energy which is detected by a plurality of microphones arranged in a radial pattern. Differences in the received energy at each microphone are evaluated to determine wind flow velocity and direction. This system is not capable of detecting a vertical wind profile. It determines only wind parameters within the flow encompassing the loudspeaker and microphones.
J. Nicoli, U.S. Pat. No. 4,174,630 for "Ultrasonic Anemometer" and Y. Kobori et al, U.S. Pat. No. 3,693,433 for "Ultrasonic Anemometer" are similar to the Pear, Jr. system in that sonic transmitters radiate energy toward receivers and the effect of wind flow through the transmission path is analyzed to provide the wind parameter data.
The early ultrasonic anemometers were improved by using echo techniques in the sonic band. One family of devices using this approach was developed by Martin Balser. It is the subject of U.S. Pat. Nos.: 3,889,533 for "Acoustic Wind Sensor"; 4,143,547 for "Method And Apparatus For Remote Measurement Of Variation In Wind Velocity"; 4,206,639 for "Doppler Acoustic Wind Sensor; and 4,558,594 for "Phased Array Acoustic Antenna". These patents illustrate a system for remote measurement of wind velocity which include an array of acoustic transducer elements adapted to provide a beam of acoustic energy along a path and detect the acoustic energy that has been scattered by the wind in the beam path. The systems require a complex switching scheme to alternately connect the transducers between the transmitter and receiver portions of a system.
P. MacCready, Jr., U.S. Pat. No. 4,219,887 for "Bistatic Acoustic Wind Monitor System" and E. Brown, U.S. Pat. No. 4,481,517 for "Echometry Device And Method" illustrate further improvements. In these systems, an acoustic pulse is transmitted from the ground upwards along a beam path and a transmitter receives echoes resulting from atmospheric scatter along a second path. In MacCready, the transmission beam is vertical and the reception path is at an angle to the vertical while Brown illustrates a system where the transmission path is at an angle to the vertical and the receiver path is vertical. The Brown system is utilized to detect temperature variations rather than wind profile velocities.
M. Hurtig et al, U.S. Pat. No. 4,573,352 for "Apparatus For Measuring Wind Speed And/Or Direction" issued March 4, 1986 discloses a three beam approach to determining wind profile data. The system incorporates three separate transducers and associated parabolic reflectors to create three beams.
A system whose signal generation is based on the principles of linear acoustics, as do all of the patent references cited above, faces an unsatisfying engineering compromise in the choice of operating frequency. Attenuation of acoustic energy propagating in the atmosphere varies approximately as the square of the frequency. Therefore, for penetration (maximum range or maximum height) in excess of a few hundred meters, it is necessary to use a low acoustic frequency. But, as the frequency decreases, the wavelength increases and the atmosphere becomes more transparent to the acoustic energy. Of greater significance, in order to achieve adequate spatial resolution, it is necessary to produce a sharp beam (typically on the order of 10 degrees or less in width) and this requires that the linear acoustic array be many wavelengths in diameter (7 wavelengths for a 10 degree beamwidth). Thus, a linear acoustic source array with a sharp beam can become ungainly in size, for example, a system operating at 500 Hz with a 10 degree beamwidth, would have an array diameter of 4.7 meters (15.4 feet). Because of this, most acoustic sounding systems have an operating frequency above 1 kHz. For these reasons, acoustic systems which can measure wind profiles to heights of several kilometers are not likely to be based on linear acoustical principles. However, a system based on the generation of sound by the principles of non-linear acoustics, can achieve simultaneously a sharp beam and deep penetration. Such a system will be described below.
M. Fukushima et al, U.S. Pat. No. 4,351,188 for "Method And Apparatus For Remote Measurement Of Wind Direction And Speed In The Atmosphere" is exemplary of systems for determining wind profile which utilize a combination of acoustic transmitters and radio wave transmitter and receivers. Systems such as this produce accurate wind profile data but are extremely complex and difficult to implement in the environment of most modern major airports.
The system unfolded by the specification and drawing of this patent and defined by the claims presented herein are structured around certain physical truths or principles which are presented in the following references.
1. Horton, J. Warren, "Fundamentals of Sonar," Vol. 3, U.S. Navy Underwater Sound Laboratory, New London, Conn. (1955). PA0 2. Little, C. G., et al, "Remote Sensing of Wind Profiles in the Boundary Layer," ESSA/U.S. Dept. of Commerce, ESSA TR ERL 168-WPL 12 (1970). PA0 3. Little, C. Gordon, "Prospects for Acoustic Echo Sounding" Chapter 19 of "Remote Sensing of the Troposphere," (V. E. Derr, Ed.), NOAA/U.S. Dept. of Commerce & Wave Propagation Laboratory/Univ. of Colo. (1972). PA0 4. Urick, Robert J., "Principles of Underwater Sound," McGraw-Hill (1975). PA0 5. Neff, William D., "Quantitative Evaluation of Acoustic Echoes from the Planetary Boundary Layer," NOAA/U.S. Dept. of Commerce, NOAA TR 332-WPL 38 (1975). PA0 6. MacWilliams, F. J. & N. J. A. Sloane, "Psuedo-Random Sequences & Arrays," Proc. IEEE 64 (1976) pp. 1715-1729. PA0 7. Clay, Clarence S. & H. Medwin, "Acoustical Oceanography: Principles & Applications," Wiley-Interscience (1977). PA0 8. Dixon, Robert C., "Spread Spectrum Systems," Wiley, N.Y. (1976) pp. 53-91
The preceding publications are incorporated herein by reference and cited throughout the specification to precisely relate their teachings to the present invention.